1. Field of the Invention
The invention relates to a variable speed fan motor which is mainly used in civil heating and ventilation air-conditioning (HVAC) systems.
2. Description of the Related Art
A typical variable speed fan motor in a HVAC system employs a fixed-speed or multi-speed permanent-split capacitor (PSC) motor which includes at least two independent power connectors to demodulate a heating or cooling mode of operation. A power input end of the heating or cooling is usually connected to different winding taps of the PSC motor, which provides different operating speeds for fans in different operational modes. Thus, such PSC motors have low efficiency.
Due to the low efficiency of the PSC motors, many updated HVAC systems employs variable speed motors such as brushless permanent-magnet motors (BPM) and corresponding variable speed motors with electronic controllers. The rotational speed of BPM can be controlled by electronic components, which is particularly set to produce required air flows matching with each application. In addition, the power consumption of BMP is approximately in direct proportion to a cube of a rotational speed of the motor, and the electrical work of the PSC motor is approximately in direct proportion to the rotational speed of the motor. Thus, when the motor speed drops, the electrical work of BPM within the whole scope of motor speed is lower than that of BPM. This is especially important when the fan runs continuously and repeatedly.
The BPM employed in the HVAC system has the advantages of variable speed motors but doesn't change the original HVAC system obviously; it lowers the complexity of the HVAC system by using simple control circuits and cancels additional arrangements of wires which are usually used in connecting the regular variable speed motors and the existing alternative variable speed motors. However, the existing variable speed fan controllers which are used to replace PSC motors have the following defects:                1) Current transformers and related circuits are applied in detecting the status of each power input line. The applied current transformers have defects of big size, high cost, severe electromagnetic interference, large space occupation in motor controllers, bad heat dissipation of motor controllers and difficult wiring and installation procedures.        2) A plurality of sensing units of current transformers are imbedded in the connecting plug of the motor, which is difficult in installation, low in production efficiency, and disadvantageous for mass production.        